The invention relates to an inerting method that can be applied to the production of plastic articles by extrusion or injection moulding.
The extrusion or injection moulding of plastic is a known process which consists in melting a polymer and in continuously feeding the polymer melt into a die or injection-moulding head in order to form it. In general, this process employs:                a polymer feed hopper into which the polymer is introduced in powder or granule form;        means for transporting, heating and mixing the polymer coming from the hopper, for example a heated feed screw;        an extrusion head consisting of a die head giving the polymer the desired shape or a head for injecting it into a mould.        
It is known that during this type of process, and especially during the melting step in the feed screw, certain polymers may be sensitive to the presence of oxygen, the latter degrading the final properties of the formed plastic (yellowing, cross-linking of the polymer, variation in the molecular mass). The Applicant has also found that oxygen results in:                fouling of the forming device, which makes it necessary for the production tool to be cleaned, and therefore stopped, regularly, hence resulting in high maintenance costs and in loss of production;        the presence of deposits on the surface of the formed polymer.        
To remedy the problem of degradation of the final products of the formed plastic, it is known to add antioxidants to the initial polymer. However, these additives considerably increase the production cost of the plastic. Another approach in the prior art consists in forming the polymer in the presence of an inert gas. This approach was, for example developed in Application EP-A1-0 760 278. According to the method described in that application, the best inerting results could be obtained only by introducing the inert gas at a precise point on the feed screw. In the case of an existing industrial plant, introducing inert gas into the feed screw may prove to be complicated, or even impossible, as it is necessary to drill the cylindrical body of the screw without this drilling disturbing the subsequent operation of the screw. Furthermore, this preferred implementation allowed the residual oxygen content in the nitrogen to be lowered by a value of only 1% by volume.